Spread spectrum CDMA communications is presently being used for a number of commercial applications and is expected to proliferate as the demand for untethered communications increases. One example of this art is the IS-95 standard which exists for cellular telephony. In this case, the base station of this star configured network transmits a set of orthogonal Walsh functions which are overlaid with a pseudo-noise (PN) sequence. Each orthogonal function carries voice or data for a single user. See M. J. E. Golay, IDA Report 108, Pg. 110 (1965) which discloses this basic signal format. Also see Magill et al. "Spread Spectrum Technology for Commercial Applications", IEEE, June 1994, incorporated herein by reference.
It is well known that direct sequence spread spectrum CDMA systems frequently use either bi-phase PN modulation and bi-phase shift keyed (BPSK) data modulation as in the GPS system, or quadraphase (QPSK) PN modulation and BPSK data as in the IS-95 cellular telephone standard. Also see Gilhousen et al. U.S. Pat. No. 5,103,459. This is because the use of a higher order data modulation such as QPSK would make the system more complex without any bandwidth improvement or increase in processing gain.
The object of the present invention is to provide an orthogonal code division multiple access (OCDMA) communication system having an improved modulation system providing increased bandwidth efficiency.